Field Guide · term

Also known as: AMBE-3000 packet, DVSI vocoder packet, AMBE-3003 wire format

The AMBE-3003 packet is the serial wire format that carries data to and from a DVSI AMBE-3000/3003 hardware vocoder chip — the physical DSP device that encodes and decodes AMBE+2 speech. When GopherTrunk is built to offload vocoding to such a dongle (behind the dvsi build tag), it speaks this framing over the chip’s serial link: a fixed sync byte, a length, a type, and a payload.1 The packet framing is the only part of the hardware path that is pure protocol — no MBE math — so it compiles unconditionally, while the code that actually drives the chip is gated behind the build tag.

sync0x61 length2 bytes, BE type1 byte payload length covers type + payload
Every AMBE-3003 packet opens with the 0x61 sync byte, a big-endian length covering the type byte plus payload, a type byte, then the payload.

Wire format

The header is four bytes: the sync byte 0x61, a two-byte big-endian length, and a one-byte type. The length field covers the type byte plus the payload, so a packet with no payload has length 1, and the full packet on the wire is three header bytes plus the declared length. A decoder validates the sync byte first, then checks that the buffer length matches the declared length exactly — trailing bytes are rejected rather than silently accepted, which keeps a byte-stream resync honest. A SplitPackets helper walks a stream of back-to-back packets and returns the leftover trailing bytes of an incomplete packet for buffering across the next read, stopping on the first bad sync so the caller can resync upstream.

Packet types

The one-byte type field names the packet’s purpose:

Type Value Payload
PktControl 0x00 configuration / status exchange (sample rate, channel format)
PktChannelData 0x01 one 49-bit AMBE+2 frame, packed in 7 bytes
PktSpeechData 0x02 160 samples of 16-bit signed PCM (320 bytes, little-endian, 8 kHz)
PktAck 0x06 the chip’s response to a control request

The two data types are the working pair. A PktChannelData packet carries the compressed 49-bit AMBE+2 frame — from host to chip for synthesis, or from chip to host for encoding. A PktSpeechData packet carries the decompressed side: one 20 ms voice frame as 160 samples of little-endian 16-bit PCM at 8 kHz mono, 320 bytes. Control packets configure the chip and draw an ack in reply.

Why the framing is unconditional

The framing primitives — encode, decode, split — carry no patent surface, because describing a chip’s serial wire protocol is not the same as implementing the proprietary vocoder algorithm. So GopherTrunk compiles them into every build, unconditionally, and keeps them unit-testable without any hardware present. Only the Vocoder that opens the FTDI serial link and streams packets to the physical chip sits behind //go:build dvsi. This split lets the packet format be reasoned about, fuzzed, and round-tripped in ordinary CI while the hardware-dependent transport stays optional.

Relevance to SDR

Most GopherTrunk deployments decode AMBE+2 in pure Go and never touch a chip. The AMBE-3003 packet path exists for the cases where an operator has a DVSI dongle and prefers to offload vocoding to it — for throughput, for bit-exact conformance against DVSI’s own silicon, or because a build cannot use the software decoder. In that mode GopherTrunk becomes a packet pump: it pulls 49-bit AMBE+2 frames out of the P25 Phase 2, DMR, or NXDN bitstream, wraps each in a PktChannelData packet, and reads back PktSpeechData PCM to feed the audio pipeline. The framing is small, but it is the contract between GopherTrunk and the hardware, so its length and sync handling have to be exact.

Sources

  1. Digital Voice Systems — Wikipedia, on DVSI, the AMBE vocoder family, and its hardware vocoder products. ↩

See also